The test results so far have been in line with the testing Boeing did when it was developing the fix, spokesman Marc Birtel said on Wednesday.
Smoldering batteries - including one fire on the ground - prompted air safety authorities to ground the world's 787s in mid-January. Boeing has developed what it believes is a permanent fix, including more heat insulation and a system for venting battery gases outside of the plane.
The tests under way now are aimed at demonstrating that the fix worked. Ultimately, the Federal Aviation Administration will have to certify the changes before the planes fly again.
Ground testing is continuing at Boeing labs in Seattle. Boeing is giving results to the FAA as they are finished.
The testing has been taking longer than Boeing had originally suggested when it announced the fix for the 787's smoldering batteries on March 15.
"We are all working toward returning the 787 fleet to service in the most expeditious manner possible, but we are being thorough and won't shortcut the test and certification process," Birtel said in a written statement.
Boeing said a 787 test flight on Wednesday was unrelated to testing its battery fix. The flight was a "routine test flight designed to address some of the component reliability projects" it has been working on, the company said. A demonstration flight for the battery fix "will take place in the coming days," Boeing said.
Meanwhile new orders for the 787 had nearly dried up since the plane was grounded. On Wednesday, though, International Airlines Group said it would convert options for 18 787s into firm orders for its British Airways unit, according to a securities filing in Spain. The planes will replace 747-400s starting in 2017.
The company also said it has agreed to terms that could lead to an additional 787 order for its Spanish airline, Iberia. The deal won't turn into a firm order until Iberia restructures and reduces expenses "and is in a position to grow profitably," IAG said.
The only other 787 order Boeing has booked this year is for 42 planes for American Airlines. That deal was reached in 2011 but needed bankruptcy court approval.
Before the announcement Wednesday, Boeing had a backlog of orders for 841 of the planes. It is continuing to build them, even though it can't deliver them now. With planes still rolling off production lines, 15 have been added to its inventory, on top of about 41 planes in inventory at the end of last year, UBS analyst David Strauss wrote on Monday.
The National Transportation Safety Board plans a hearing into the battery problems on April 23 and 24th.
Boeing Co. shares rose 27 cents to close at $84.36 on Wednesday.